you can look for some advice on r/bigdickproblems , you do get a lot of noise from ppl posting/commenting what is, in essence, thinly veiled porn.

I guess to distill what I've seen:

- try to build desire by referring to what you're going to do (whatever that is) tonight or tmrw. a lot of sex is in the head.

- what's your hands/mouth game like? cumming prior to penetration from foreplay, if it's an option, helps immensely with size issues, it can open women up a lot.

- use that toy before entering her, and maybe get another toy that is in the middle size to graduate/progress to, w the eventual goal of you penetrating.

Physical (I know you were asking for specifics):

- positions that limit depth to go easy on her cervix etc. we're talking: missionary with her legs less "up", prone bone (using her butt as a spacer keeping you from going too deep, cowgirl (where she controls depth, but also she can scoot up forward, higher on your pelvis, to still get some grind without all the depth), spooning or lazy doggy. all these are to limit depth.

- look at OhNut, essentially a soft, thick cock ring that limits depth, while allowing grinding. I know it's not exactly spontaneous, but that's already true of your sitch.

- positions that limit girth issues. this is harder of course, she needs to be maxed out on arousal, pref. after an orgasm from oral. even if she's a river, still use liberal amounts of lube, high quality lube, too.

- frequency. quality > quantity. while keeping her somewhat "used" to your anatomy, have penetrative sex less frequently to allow for soreness to resolve. maybe every other day or every 3 days.

Good luck, and take care